1. Field of the Invention
The present invention relates generally to a lock-up control system for an automatic power transmission of an automotive vehicle. More specifically, the invention relates to a pressure control system for a lock-up clutch for establishing and releasing a lock-up state with compensation of fluid pressure particularly at low engine speed range.
2. Description of the Background Art
One example of a conventional lock-up clutch pressure control system has been disclosed in "Jidosha Kogaku Extra Edition Vol. 37, No. 7", published by Tesudo no Nihon, June, 1989, pages 214 to 249. On page 221 of this publication, a working fluid discharged from an oil pump is introduced into a torque converter via a pressure regulator valve for generating a torque converter pressure. The working fluid is also used for controlling a lock-up clutch as introduced into a lock-up control fluid circuit which includes a lock-up control valve. An extra flow rate of the working fluid is introduced into a lubricating circuit which supplies the working fluid to components to be lubricated as lubricant.
However, in such prior proposed pressure control system, all of extra flow rate of the working fluid is relieved into the lubricating circuit. A low engine speed range, the discharge rate of the oil pump can become insufficient. In such occasion, because of leakage through the fluid circuit including the amount used for lubrication, the lock-up clutch actuation pressure can become insufficient. This results in prolongation of a clutch engaging and disengaging transition period when a transmission speed ratio is shifted. Particularly, this tendency will be increased while the working fluid temperature is relatively high. Especially, such lag time will creates problems at drive range selection N (neutral) range or P (parking) range to R (reverse) range or D (automatic shifting) range. Namely, during the lag dime with incomplete engagement or delay of engagement of the lock-up clutch, durability of the clutch can be reduced. Furthermore, due to incomplete engagement or delay of engagement, no load acceleration of the engine or an uncomfortable vibration of the vehicular body can be caused for degradation of vehicular drivability.
Such defects may be improved by providing a greater discharge rate for the oil pump even at low engine speed range. However, increasing of discharge rate of the oil pump necessarily causes increasing of the engine load for degradation of fuel economy. Particularly, since the discharge rate of the oil pump at a relatively high speed range will never become insufficient. An increased discharge rate at the low engine speed range simply increases the wasting of engine output at high speed range.